Sequential stepped directional control valve

ABSTRACT

A sequential stepped directional control valve accepts pressurized hydraulic fluid from a pressure port, and directs that pressurized fluid to a first stage port, or to both a first stage port and a second stage port. In the neutral position, the valve blocks the flow of pressurized hydraulic fluid into the valve. In the first stage position, pressurized hydraulic fluid flows into the valve and is directed by the valve to the first stage port, however, the fluid flow to the second stage port is blocked. In the second stage position, pressurized hydraulic fluid flows into the valve and is directed to both the first and second stage ports. When the valve transitions from the first stage position to the second stage position, the valve does not cycle through a neutral position, and the flow of pressurized hydraulic fluid to the first stage port is uninterrupted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from and is related to the followingapplication, Sequential Stepped Directional Control Valve, U.S.Provisional Application No. 60/968,114, filed Aug. 27, 2007. The priorapplication, including the entire written description and drawingfigures, is hereby incorporated by reference into the presentapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to three position hydraulicvalves used in a wide variety of hydraulic applications. Hydraulicvalves of this sort may be utilized, for example, to cause pistonmovement and thereby cause operation of hydraulic equipment. One exampleof such an application would be engine brakes. The present inventionrelates to an improved design for such hydraulic valves.

2. Brief Description of the Related Art

Prior art or traditional three position valves utilized two solenoidcoils, with one solenoid associated with a pull pole piece and onesolenoid associated with a push pole piece. A plunger was orientedbetween the two pole pieces with the neutral position for the plungerbeing mid-stroke between them. One or more return springs were providedfor returning the plunger to the neutral mid-stroke position between thepush and pull pole pieces.

When electrical current was applied to one of the solenoid coils, amagnetic force was created in the associated pole piece (either the pushpole piece or the pull pole piece, respectively) which caused themagnetically responsive plunger to attempt to close the air gap betweenthe magnetically operational pole piece and the plunger, causing thereturn spring or springs to compress. When the electrical current wasremoved from the solenoid, the magnetic force in the operational polepiece was removed, and the springs returned the plunger to the neutralmid-stroke position. The plunger movement was, in turn, mechanicallytransmitted to a spool type directional control valve, which, directedhydraulic fluid flow. On a traditional two coil, three position valve,the valve was required to move through a neutral position in order totransition from a first operating position (e.g., with the plungermagnetically attracted to push pole piece) to a second operatingposition (e.g., with the plunger magnetically attracted to pull polepiece).

The present invention, called a sequential stepped directional controlvalve, overcomes the problems associated with traditional three positionvalves. The present invention does not involve transitioning through anon-operational neutral position in order to change from a first stageoperational position to a second stage operational position. Dependingon the nature of the hydraulic application, such a transition through aneutral position, as described in more detail below, can be highlydisadvantageous, dangerous, or even deadly. The present invention,moreover, is economical, and involves less parts that can lead tofailure. Whereas traditional prior art three position hydraulic valvesrequired two solenoids, and two pole pieces (a push and pull polepiece), the present invention requires only a single solenoid and asingle pole piece, saving costs and decreasing complexity. Byeliminating the extra traditional parts, the present invention canachieve a savings in size as well.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present embodiments ofthe invention described herein to provide a hydraulic valve, called asequential stepped directional control valve, that overcomes theshortcomings of the prior art traditional three position hydraulicvalves, while still achieving the functions and benefits of priorsystems.

In particular, it is an object of the embodiments of the invention toachieve a linear sequential stepped directional control valve thatutilizes a single solenoid, a single plunger, and a single pull polepiece, biased by multiple sequenced preloaded springs to achievemultiple operating positions.

It is a further object of the embodiments of the invention to controlthe stages or operating positions of the valve of the present inventionby controlling the electrical current applied to the single solenoid byapplying the desired magnitude of electrical current required for thesequential stage or position.

It is another object of the embodiments of the invention to provide asequential stepped directional control valve that is capable oftransitioning from a non-neutral first stage (wherein hydraulic fluidunder pressure is directed to a first stage port for utilization by afirst hydraulic application) to a non-neutral second stage (whereinpressurized hydraulic fluid is directed, in addition, to a second stageport for utilization by a second hydraulic application) withouttransitioning through a neutral stage (wherein pressurized hydraulicfluid is not directed to the first stage port or the second stage port).

Still another object of the embodiments of the invention describedherein is to achieve proportional control of the sequential steppeddirectional control valve by, inter alia, the application of Pulse WidthModulated current to the solenoid of the valve.

A still further object of the present embodiments of the inventiondescribed herein is to achieve the above objects in a manner that iscost-efficient.

Yet another object of the embodiments of the invention described hereinis to achieve the foregoing objects while decreasing the size or weightof the valve, as compared to prior art valves.

The disclosed embodiments of the sequential stepped directional controlvalve achieve the aforementioned objects, and others, because theyinclude features and combinations not found in prior art valves, and, inparticular, not found in prior art three position hydraulic valves.

In the described embodiments of the present invention, an improvedhydraulic valve, called a sequential stepped directional control valve,is provided, wherein the valve is inserted into a valve bore in the bodyof an application (hydraulically operated equipment). The valve bore isin hydraulic communication with a pressure port (hydraulically connectedto a source of pressurized hydraulic fluid), a tank port (hydraulicallyconnected to an unpressurized hydraulic fluid tank), a first stage port(hydraulically connected to a first hydraulic application), and a secondstage port (hydraulically connected to a second hydraulic application).

The principal embodiment of the sequential stepped directional controlvalve described herein has three stages or positions: a neutral stage, afirst stage, and a second stage. Pressurized hydraulic fluid is providedthrough the pressure port to the sequential stepped directional controlvalve. In the neutral stage, an axially moving spool within the valve isbiased to be in the neutral position, preferably by two springs. In theneutral position, the spool blocks the pressurized hydraulic fluid fromentering the sequential stepped directional control valve, and thepressurized hydraulic fluid is not directed by the valve to either thefirst stage port or the second stage port. Instead, the spool in theneutral position permits hydraulic fluid from both the first stage portand the second stage port to enter the valve and drain through thehollow central spool passage in the spool to the unpressurized tankport, relieving hydraulic pressure in both the first stage port and thesecond stage port. In the absence of pressurized hydraulic fluid, thehydraulic applications connected to the first stage port and the secondstage port do not operate.

When a first predetermined electrical current is applied to the solenoidin the sequential stepped directional control valve, a magnetic pullingforce is created via a pull pole piece which pulls a plunger attached tothe spool partway toward the pull pole piece, partially overcoming theforce applied to the plunger by the valve's springs, to a first stageposition. In the first stage position, the spool has moved to a positionwherein the spool no longer blocks hydraulic communication between thevalve's interior and the pressure port, so as to permit pressurizedhydraulic fluid from the pressure port to enter the sequential steppeddirectional control valve. The spool directs the pressurized hydraulicfluid to the first stage ports via a first spool cavity which, in thefirst stage position, is simultaneously in hydraulic communication withboth pressure port and the first stage port. In the first stageposition, the spool permits hydraulic fluid to flow from the secondstage port, draining through the hollow central spool passage in thespool to the unpressurized tank port, thereby relieving hydraulicpressure in the second stage port. Because pressurized hydraulic fluidhas been directed to the first stage port when the spool is in the firststage position, the hydraulic application connected to the first stageport is capable of operation. Conversely, because the second stage portis relieved of hydraulic pressure, the hydraulic application connectedto the second stage port is inactivated.

By applying a second (greater) predetermined electrical current to thesolenoid, the magnetic pulling force in the pull pole piece isincreased, and the plunger attached to the spool is pulled closer to thepull pole piece, further overcoming the force exerted on the plunger bythe valve's springs, to a second stage position. In the second stageposition, and throughout the transition from the first stage position tothe second stage position, the spool is in a position that allowspressurized hydraulic fluid to enter the sequential stepped directionalcontrol valve, and the spool directs that pressurized hydraulic fluid tothe first stage port via the first spool cavity which is simultaneouslyin hydraulic communication with both the pressure port and the firststage port. Also in the second stage position, the spool directspressurized hydraulic fluid to flow from a first stage cavity adjacentthe first stage port to the second stage port via a second spool cavity.In the second stage position, the second spool cavity is simultaneouslyin hydraulic communication with the first stage port (which haspressurized hydraulic fluid directed to it from the pressure port by thefirst spool cavity) and the second stage port. Thus, pressurizedhydraulic fluid is directed to both the first stage port and the secondstage port. At the same time, the spool is positioned so as to preventpressurized hydraulic fluid from either the first stage port or thesecond stage port from draining into the hollow central spool passage inthe spool to the unpressurized tank port. Because the sequential steppeddirectional control valve has directed pressurized hydraulic fluid toboth the first stage port and the second stage port, the hydraulicapplications connected to the first sage port and the second stage portare both capable of operation.

Importantly, because the sequential stepped directional control valvecontinued to direct pressurized hydraulic fluid to the first stage portthroughout (and after) the transition of the spool from the first stageposition to the second stage position, the first stage application wasoperational throughout the transition. Stated another way, intransitioning from the first stage position (with pressurized hydraulicfluid directed to the first stage port) to the second stage position(with pressurized hydraulic fluid directed to both the first stage portand second stage port), at no time during the transition did thesequential stepped directional control valve cycle through a neutralposition (wherein no pressurized hydraulic fluid would be directed tothe first stage port or second stage port).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating an embodiment of theinvention with the spool in the neutral position.

FIG. 2 is a cross-sectional view of the embodiment of the invention ofFIG. 1 with the spool in the first stage position.

FIG. 3 is a cross-sectional view of the embodiment of the invention ofFIG. 1 with the spool in the second stage position.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the sequential stepped directional control valve 110 ofthe present invention is illustrated in FIGS. 1-3 in a manner that wouldbe understood by persons skilled in the art. For purposes of thisdescription, the sequential stepped directional control valve 110 shallhave a distal end or direction oriented in the direction of the tankport 122 and a proximal end or direction oriented in the direction ofthe portion of the sequential stepped directional control valve 110opposite from the tank port 122. The lengthwise direction of thesequential stepped directional control valve 110 illustrated by thebroken center line in FIGS. 1-3 shall be referred to generally as theaxial direction, while the radial direction of the sequential steppeddirectional control valve 110 is transverse to the axial direction.

A body 112 in which the sequential stepped directional control valve 110is utilized is illustrated in FIGS. 1-3. The body 112 may be found inany number of hydraulic applications for which stepped directionalcontrol valves are utilized, for example, in an engine brake.

The body 112 contains a valve bore 116 formed in the body in an axialdirection, into which the sequential stepped directional control valve110 is inserted. Preferably extending generally radially through thebody 112 from the valve bore 116 is the pressure port 114 which delivershydraulic fluid under pressure. The hydraulic fluid provided by thepressure port 114 is typically delivered under pressure by a hydraulicpump (not illustrated), and is pumped from a hydraulic connection to anunpressurized hydraulic fluid tank (not illustrated). A first stage port118 and a second stage port 120 also preferably extend generallyradially through the body 112 from the valve bore 116. The first stageport 118 is hydraulically connected to whatever hydraulically operateddevice is desired to be operated initially in a sequence, and the secondstage port 120 is hydraulically connected to a hydraulically operateddevice desired to be operated secondarily in a sequence, when thesequential stepped directional control valve 110 is caused to move froma neutral position (FIG. 1) to a first stage (FIG. 2) and then to asecond stage (FIG. 3), respectively. For example, the first stage port118 may be hydraulically connected to a device that provides lightengine braking, and the second stage port 120 may be connectedhydraulically to a device that provides heavy engine braking.

Referring once again to FIGS. 1-3, tank port 122 extends through thebody 112 and is connected preferably in a substantially axial directionto the valve bore 116. Tank port 122 is typically connectedhydraulically to an unpressurized hydraulic fluid tank (notillustrated).

As discussed previously, the valve bore 116 is formed in the body 112 toreceive the sequential stepped directional control valve 110, which hasa hollow, somewhat cylindrically shaped cage 124 that forms the outsidesurface of the portion of the sequential stepped directional controlvalve 110 that is inserted into the body 112. Formed in the interior ofthe cage 124 is an axially extending cage central passage 136. Cage 124has a proximal end 126 and a distal end 128. Formed at the distal end128 of the cage 124 is a cage tank port 130 connecting hydraulically tothe hollow cage central passage 136. The cage tank port 130 connectshydraulically to tank port 122.

Located in the valve bore 116 adjacent to the pressure port 114 is anannular pressure port cavity 132 encircling the cage 124. The pressureport cavity 132 is formed between the inner wall of the valve bore 116and the outer wall of the cage 124. Formed through the wall of the cage124 adjacent the pressure port cavity 132 are one or more cage pressureports 134 that are capable of permitting hydraulic fluid flow betweenthe pressure port cavity 132 and the hollow interior cage centralpassage 136.

Also located in the valve bore 116, but adjacent to the first stage port118 is an annular first stage cavity 138 encircling the cage 124. Thefirst stage cavity 138 is formed between the outer wall of the cage 124and the inner wall of the valve bore 116. One or more cage first stageports 140 are formed through the wall of the cage 124 adjacent the firststage cavity 138 that are capable of permitting hydraulic fluid flowbetween the first stage cavity 138 and the cage central passage 136.

In addition, located in the valve bore 116 adjacent to the second stageport 120 is an annular second stage cavity 142 encircling the cage 124,formed between the outer wall of the cage 124 and the inner wall of thevalve bore 116. One or more cage second stage ports 144 are formedthough the wall of the cage 124 adjacent the second stage cavity 142that are capable of permitting hydraulic flow between the second stagecavity 142 and the cage central passage 136.

Located on the outside wall of the cage 124, between the tank port 122and the second stage cavity 142, and on either side of the first stagecavity 138 and the pressure port cavity 132 are raised lands 146annularly encircling the outside wall of the cage 124. The raised lands146 have U-shaped troughs 148 for receiving resilient O-rings 150,preferably made of a resilient elastomer. When the sequential steppeddirectional control valve 110 is inserted into valve bore 116, theresilient O-rings 150 are compressed between the U-shaped troughs 148 ofraised lands 146 and the inner wall of the valve bore 116 to form atight seal that substantially prevents hydraulic fluid flow alongsidethe outside of the cage 124, that is, along the outside wall of the cage124, for example, between adjacent ports and/or cavities.

Continuing to refer to FIGS. 1-3, cage central passage 136 is formedgenerally in the shape of a hollow circular cylinder oriented in anaxial direction to receive a snugly fitting spool 152, which is capableof some movement back and forth within the cage central passage 136 inthe axial direction. Spool 152 has a hollow central spool passage 154extending substantially axially therethrough. The spool 152 has aproximal end 156 and a distal end 158. Formed through the distal end 158of the spool 152 in the axial direction is a spool tank port 160 thatallows hydraulic fluid to flow from the central spool passage 154through the cage tank port 130 to the tank port 122.

Extending radially through the wall of spool 152 are one or more spoolfirst stage ports 162 which permit hydraulic fluid to flow from the cagefirst stage ports 140 through the first stage spool ports 162 into thecentral spool passage 154 when the spool 152 is oriented in a mannersuch that spool first stage ports 162 are aligned with cage first stageports 140, as illustrated in FIG. 1.

Similarly, extending in the radial direction through the wall of spool152 are one or more spool second stage ports 164. Hydraulic fluid flowis permitted to flow from cage second stage ports 144 through spoolsecond stage ports 164 into the central spool passage 154 when the spoolsecond stage ports 164 are aligned with cage second stage ports 144, asillustrated in FIGS. 1 and 2.

Extending annularly around the spool 152, and formed between the outerwall of the spool 152 and the inner wall of the cage 124, that is, thewall of the cage central passage 136, is a first spool cavity 166 and asecond spool cavity 168. Depending upon the orientation of the spool 152within the cage 124, the first spool cavity 166 and the second spoolcavity 168 may allow hydraulic fluid to flow: (1) from the pressure port114 through the pressure port cavity 132 through cage first stage ports140 into the aligned first spool cavity 166 out through the cage firststage ports 140 into the first stage cavity 138 and to the first stageport 118, when the spool 152 is in the first stage position or secondstage position, as illustrated in FIGS. 2 and 3; and (2) from the firststage cavity 138 into the cage first stage ports 140 through the alignedsecond spool cavity 168 out through the cage second stage ports 144 tothe second stage cavity 142 to the second stage port 120, when the spool152 is in the second stage position, as illustrated in FIG. 3.

Attached to the proximal end 156 of spool 152 is plunger 170, which ismagnetically responsive to a magnetic field generated through pull polepiece 174 when an electrical current is supplied to solenoid 172, whichsubstantially annularly surrounds the pull pole piece 174. When anelectrical current is provided to solenoid 172, pull pole piece 174generates a magnetic force that pulls plunger 170 (and, hence,mechanically connected spool 152) toward the pull pole piece 174. Astronger electrical current applied to the solenoid 172 generates astronger pulling force in pull pole piece 174 that pulls plunger 170more towards the pole piece 174; conversely, a weaker electrical currentapplied to the solenoid 172 generates a weaker pulling force in pullpole piece 174 acting upon plunger 170, hence pulling it less.

Referring to FIG. 1, plunger 170 has a hollow plunger central passage176. Plunger central passage 176 preferably accommodates the proximalend 156 of spool 152. Within the central spool passage 154 is secondstage spring 178, preferably being a compression spring. The distal endof second stage spring 178 fits against second stage spring detent 180.Fitting within the hollow axial center of second stage spring 178 isgenerally pin-shaped spring stop 182. Spring stop 182 has a spring stopshoulder 184, preferably formed as an annular projection around springstop 182 between the two ends of spring stop 182. Spring stop shoulder184 restrains the proximal end of second stage spring 178, and receivesthe preload force imparted by second stage spring 178, transmitting thatforce to spring stop 182. Second stage spring 178 is preferably somewhatcompressed between second stage spring detent 180 and spring stopshoulder 184, and therefore imparts a spring force on both. In theneutral position illustrated in FIG. 1, this spring force exerted bysecond stage spring 178 causes spring stop shoulder 184 to abut springstop detent 186. Spring stop shoulder 184 is received within plunger170. The proximal end 156 of spool 152 preferably effectively formsdetent for spring stop shoulder 184, preventing excess movement of thespring stop 182 in the distal direction relative to the plunger 170.Plunger central passage 176 has formed within it a spring stop detent186 preventing excess movement of the spring stop 182 in the proximaldirection relative to the plunger 170.

On the proximal side of plunger 170, plunger central passage 176preferably accommodates at least a portion of first stage spring 188,preferably a compression spring. Spring stop 182 extends at leastpartially through the hollow axial center of first stage spring 188. Thedistal end of first stage spring 188 abuts against the opposite side ofspring stop shoulder 184 from second stage spring 178. Preferably, firststage spring detent 190 is formed as a cavity within pull pole piece174. The proximal end of first stage spring 188 is retained by firststage spring detent 190. First stage spring 188 is preferably somewhatcompressed between first stage spring detent 190 and spring stepshoulder 184, and therefore imparts a spring force on both. Second stagespring 178 is stronger than first stage spring 188; thus, when nocurrent is applied to solenoid 172, and no magnetic force is imparted onplunger 170 by pull pole piece 174, the second stage spring 180overcomes the force imparted by first stage spring 188 on spring stopshoulder 184 and causes spring stop shoulder 184 to abut against springstop detent 186. On the other hand, because plunger 170, spring stop182, and spring stop shoulder 182 move axially together with attachedspool 152, spring force exerted by first stage spring 188 upon springstop shoulder 184 in the neutral position illustrated in FIG. 1 causesthe spool 152 to move to the maximum distal position, namely, theneutral position.

Referring to FIG. 1, spool stop 192 formed in cage 124 interacts withspool shoulder 194 formed in spool 152 to prevent the force imparted onthe spool 152 by first stage spring 188 to cause spool 152 to moveexcessively toward or past the distal end 128 of cage 124. Pole pieceshoulder 196 formed in pull pole piece 174 preferably interacts withplunger 170 to prevent plunger 170 and attached spool 152 from movingexcessively toward the proximal end 126 of cage 124.

The embodiment of the sequential stepped directional control valve 110described above and illustrated in FIGS. 1-3 operates in the mannerdescribed below.

Hydraulic fluid is provided under pressure to the sequential steppeddirectional control valve 110 via pressure port 114. Referring inparticular to FIG. 1, when the solenoid 172 is de-energized, that is,when no electrical current is applied to the solenoid 172, no magneticpulling force is generated by pull pole piece 174. Plunger 170 is notthen magnetically attracted to pull pole piece 174. With no magneticforce being imparted to overcome them, the combined spring forcesapplied by first stage spring 188 and second stage spring 178 causespool 152 to move to the neutral position illustrated in FIG. 1, thatis, they cause the distal end 158 of the spool 152 to move as far as itcan toward the distal end 128 of cage 124 until it is prevented frommoving any farther in the distal direction by the interferenceinteraction of spool stop 192 formed in cage 124 and spool shoulder 194formed in spool 152. In that neutral position, the plunger 170 is in themaximum air gap position, that is, the air gap between the pull polepiece 174 and the plunger 170 is at its maximum when the sequentialstepped directional control valve 110 is in the neutral position.

In the neutral position described above, and as illustrated in FIG. 1,hydraulic fluid under pressure is supplied by pressure port 114, fillingpressure port cavity 132, which in turn causes pressurized hydraulicfluid to flow into cage pressure ports 134. Spool 152, however, blocksthe pressurized hydraulic fluid from flowing into the cage centralpassage 136, and thereby prevents the flow of pressurized hydraulicfluid from cage pressure ports 134 to either first stage port 118 orsecond stage port 120.

At the same time, in the neutral position, hydraulic fluid in firststage port 118 is free to flow from first stage port 118 to first stagecavity 142 and into cage first stage ports 140. Because the spool firststage ports 162 are aligned with cage first stage ports 140 in theneutral position, hydraulic fluid in cage first stage ports 140 is freeto flow from cage first stage ports 140 through spool first stage ports162 into the hollow central spool passage 154 to the spool tank port 160through cage tank port 130 and then to the tank port 122. Because tankport 122 is connected to an unpressurized tank, hydraulic pressure inthe first stage port 118 is relieved.

In a similar manner, in the neutral position, hydraulic fluid in secondstage port 120 is free to flow to second stage cavity 142 and into cagesecond stage ports 144. In the neutral position, as illustrated in FIG.1, spool second stage ports 164 are aligned with cage second stage ports144, and therefore hydraulic fluid in cage second stage ports 144 flowsthrough spool first stage ports 162 into central spool passage 154, andthereafter through spool tank port 160 through cage tank port 130 totank port 122. The hydraulic fluid pressure in the second stage port 120assumes the pressure of the tank as well, that is, the hydraulicpressure in second stage port 120 like the first stage port 118 isrelieved. Because the hydraulic pressure is relieved in the first stageport 118 and the second stage port 120 in the neutral position, thehydraulic functions connected to the first stage port 118 and the secondstage port 120 are thereby inactivated.

Referring to FIG. 2, in order to cause the sequential steppeddirectional control valve 110 to provide pressurized hydraulic fluid toactuate the first stage application, a first predetermined current isapplied to the solenoid 172. The current in the solenoid 172 causes pullpole piece 174 to impart a magnetic pulling force upon plunger 170.Because the magnetic pulling force is stronger than the spring forceimparted by first stage spring 188, the plunger 170 is magneticallypulled toward pull pole piece 174, and the spring force exerted by firststage spring 188 is partially overcome and compresses. The pull polepiece 174 continues to pull the plunger 170 toward pull pole piece 174until the spring force in the compressed first stage spring 188 matchesthe magnetic pulling force exerted by the pull pole piece 174 upon theplunger 170. Second stage spring 178, in the compressed, preloadedposition previously described, is preferably sufficiently strong tocontinue to push spring stop shoulder 184 against spring stop detent186. The first predetermined current is selected so as to be sufficientto cause a sufficient magnetic field in pull pole piece 174 so as tocause the plunger 170 to move sufficiently close to pull pole piece 174so that the spool 152 attached to the plunger 170 moves to the firststage position described in further detail below.

In the first stage position, as illustrated in FIG. 2, pressurizedhydraulic fluid is supplied by pressure port 114, flowing throughpressure port cavity 132 into cage pressure ports 134. When the spool152 is in the first stage position illustrated in FIG. 2, the spool 152does not block the cage pressure ports 134. Instead, the spool 152 isoriented so that first spool cavity 166 is aligned with cage pressureports 134. Consequently, pressurized hydraulic fluid flows from pressureport cavity 132 through cage pressure ports 134 into first spool cavity166. First spool cavity 166 extends axially in the distal direction.When the spool 152 is in the first stage position, the first spoolcavity 166 also is in hydraulic communication with first stage ports140. First stage ports 140 thereupon allow pressurized hydraulic fluidto flow into first stage cavity 138, and then to first stage ports 118.Thus, to summarize, pressurized hydraulic fluid is supplied by pressureport 114 to cage pressure ports 134, then flows from the cage pressureports 134 into the first spool cavity 166, and then out through the cagefirst stage ports 140 to the first stage cavity 138 and into the firststage port 118. The pressurized hydraulic fluid delivered through thefirst stage port 118 from the pressure port 114 via the sequentialstepped directional control valve 110 in the first stage position causesthe first stage hydraulic function to be operable. In the first stageposition, as illustrated in FIG. 2, the spool 152 blocks the pressurizedhydraulic fluid from reaching the cage second stage ports 144, thesecond stage cavity 142, or the second stage port 120.

At the same time, and throughout the transition from the neutralposition (FIG. 1) to the first stage position illustrated in FIG. 1,spool second stage ports 164 continue to be aligned with cage secondstage ports 144 so as to permit hydraulic fluid to flow from secondstage port 120 through second stage cavity 142, then through cage secondstage ports 144, then through aligned spool second stage ports 164, andthen into the hollow central spool passage 154 to spool tank port 160through cage tank port 130 and then to the tank port 122. The hydraulicfluid in the second stage port 120 thereby continues to assume thepressure of the tank, that is, hydraulic fluid pressure in the secondstage port 120 is relieved, and the hydraulic functions connected to thesecond stage port 120 remain inactivated.

To actuate the second stage application, a second predetermined currentis applied to solenoid 172. The second predetermined current is selectedso as to be sufficient to cause a sufficient magnetic field to begenerated by pull pole piece 174 so that the spool 152 attached to theplunger 170 moves to the second stage position described in furtherdetail below.

Upon application of the second predetermined current to solenoid 172,the current in the solenoid 172 cause pull pole piece 174 to impart astronger magnetic pulling force upon plunger 170 than occurred in thefirst stage position. As illustrated in FIG. 3, the magnetic pullingforce upon plunger 170 causes the plunger 170 to be pulled to theminimum air gap position, that is, the air gap between the pull polepiece 174 and the plunger 170 is at its minimum when the sequentialstepped directional control valve 110 is in the second stage position.

Referring to FIG. 3, as the second predetermined current is appliedsolenoid 172, and as plunger 170 is magnetically pulled further towardpull pole piece 174, the proximal end of spring stop 182 abuts firststage spring detent 190, and first stage spring 178 reaches its maximumcompression. As plunger 170 continues to be magnetically pulled towardpull pole piece 174, spring stop shoulder 184 no longer abuts (itseparates from) spring stop detent 186, and second stage spring 188begins being further compressed between spring stop shoulder 184 andsecond stage spring detent 180 until the combined spring forces of firststage spring 188 and second stage spring counter balance the magneticpulling force of pull pole piece 174 upon plunger 172 as a result of thesecond predetermined current, causing the spool 152 to be in the secondstage position illustrated in FIG. 3 and described in further detailbelow.

In the second stage position illustrated in FIG. 3, once againpressurized hydraulic fluid is supplied by pressure port 114 and flowsinto pressure port cavity 132 and into cage pressure ports 134. Spool152 does not block cage pressure ports 134; instead, the first spoolcavity 166 continues to align with the cage pressure ports 134.Consequently, pressurized hydraulic fluid from cage pressure ports 134flows into first spool cavity 166. In the second stage positionillustrated in FIG. 3, first spool cavity 166 continues to be inhydraulic communication with first stage ports 140. Indeed, as can beseen by comparing FIGS. 2 and 3, the entire time that the spool 152 insequential stepped directional control valve 110 transitions from thefirst stage position (FIG. 2) to the second stage position (FIG. 3), thefirst spool cavity continues to be hydraulically connected to both thecage pressure ports 134 (and thus to pressure port cavity 132 andpressure port 114) and to the cage first stage ports 140 (and thus tofirst stage cavity 138 and first stage port 120).

Thus, while the spool 152 is in the second stage position, and duringthe entire time that the spool 152 is transitioning from the first stageposition to the second stage position, pressure port 114 is supplyingpressurized hydraulic fluid to first stage port 118, flowing from thepressure port 114 through the pressure port cavity 132 through the cagepressure ports 134 into first spool cavity 166, then out from firstspool cavity 166 through the cage first stage ports 140 into first stagecavity 138 and into first stage port 114, where it causes the hydraulicapplication connected to first stage port 114 to operate. Becausepressurized hydraulic fluid is supplied continually to the first stageport 118 in the second stage position, in the first stage position, andthe transition from the first stage to the second stage, the first stageapplication is capable of operation throughout those stages and thetransition between them.

Referring again to FIG. 3, in the second stage position, second spoolcavity 168 is oriented so that it permits hydraulic flow to it fromfirst stage cavity 138 via the cage first stage ports 140, with whichthe second spool cavity 168 is aligned. As previously discussed, in thesecond stage position, first stage cavity 138 has pressurized hydraulicfluid supplied to it. At the same time, in the second stage position,second spool cavity 168 is oriented so that hydraulic fluid can flowfrom it to second stage cavity 142 (and thus to second stage port 120)via the cage second stage ports 144 with which the second spool cavity168 is aligned. Thus, in the second stage position, pressurizedhydraulic fluid in the first stage cavity 138 is in hydrauliccommunication with cage first stage ports 140. Pressurized hydraulicfluid therefore flows into aligned second spool cavity 168, and thenfrom second spool cavity 168 out through cage second stage ports 144into second stage cavity 142 and into second stage port 120 to renderthe hydraulic application that is hydraulically connected to secondstage port 120 operational.

When the spool 152 is in the second stage position illustrated in FIG.3, spool first stage ports 162 and spool second stage ports 164 areoriented in a manner so that the wall of the cage 124 blocks the flow ofpressurized hydraulic fluid to or through spool first stage ports 162and spool second stage ports 164. Consequently, there is no hydraulicpath for hydraulic fluid to flow from first stage port 118 to tank port122, or from second stage port 120 to tank port 122, that is,pressurized hydraulic fluid cannot flow via spool first stage ports 162or spool second stage ports 164 through central spool passage 154 outthrough spool tank port 160, because the wall of the cage 124 blocks thepassage through spool first stage ports 162 and spool second stage ports164. As a result, the hydraulic pressure in the hydraulic fluiddelivered to first stage port 118 and to second stage port 120 is notrelieved, as it would have been if the pressurized hydraulic fluidflowed between them and the tank port 122.

Importantly, as described above and as illustrated in FIGS. 2 and 3, asthe sequential stepped directional control valve 110 of the presentinvention transitions from the first stage position to the second stageposition, at no time is the flow of pressurized hydraulic fluid to thefirst stage port 118 discontinued or interrupted, that is, at no timeduring the transition does the sequential stepped directional controlvalve 110 transition through a neutral position.

This is a unique and important feature of the present invention. Assume,for example, that sequential stepped directional control valve 110 ofthe present invention was connected hydraulically to an engine brakewith the first stage port 118 being hydraulically connected to a lightengine brake function, and the second stage port 120 being hydraulicallyconnected to a heavy engine brake function. In the neutral position, noengine braking would be applied. If the equipment operator chose tolightly brake the engine, the first predetermined current would beapplied to the solenoid 172 in the sequential stepped directionalcontrol valve 110 causing pressurized hydraulic fluid to flow throughthe first stage port 118. Light engine braking would occur.

If an emergency were then to arise, requiring the operator to applyheavy engine braking, it would be disadvantageous (and potentiallydangerous, even deadly) for the operator to be required to transitionthrough a neutral (i.e., non-braking) valve cycle prior to achievingheavy braking, as was required in prior art valves. Instead, with thepresent invention, when the second predetermined current was applied tothe solenoid 172, the hydraulic fluid pressure to the first stage port118 (resulting in light engine braking) would be maintained the entiretime until (and after) the spool 152 transitioned to the second stageposition, resulting in pressurized hydraulic fluid being delivered tothe second stage port 120 (resulting in heavy engine braking being addedto the light engine braking). No neutral, non-braking condition would berequired during the transition, and thus safer engine braking would beachieved due to the uninterrupted braking.

Persons skilled in the art will recognize that the uninterrupted flow ofpressurized hydraulic fluid to the first stage port 118 while the spool152 transitions from the first stage position to the second stageposition (at which time the flow of pressurized hydraulic fluid to thesecond stage port 120 commences) is advantageous, or even necessary, ina wide range of hydraulic applications other than the engine brakeexample provided herein.

Skilled practitioners will also recognize that, while the embodimentdescribed and illustrated herein is a two stage sequential steppeddirectional control valve 110, alternative embodiments, includingadditional stages, can be added to the valve without departing from thescope of the invention. As additional stages are added, the total numberof stages is only limited by the amount of current that can be driven tothe solenoid 172 and the design of the spool type directional valve.Skilled practitioners will recognize that each additional sequentialstepped directional control position would require an additionalsequential bias spring and stop, preloaded with increasing load perstage or operating position.

Referring to FIGS. 1-3, the embodiment of the invention illustratedand/or described herein has multiple cage pressure ports 134, cage firststage ports 140, cage second stage ports 144, spool first stage ports162, and spool second stage ports 164 extending in multiple radialdirections from the cage 124 and the spool 152, respectively, formedaround the axial center line illustrated by the broken line. While thisis preferred, persons skilled in the art will recognize that either one,or more than one of the above-described parts may be used in thesequential stepped directional control valve 110 without departing fromthe scope of the invention described herein.

In the above-referenced embodiments, the electrical current applied tothe solenoid 172 to generate a magnetic pulling force in pull pole piece174 would be direct current. Additionally, persons skilled in the artwill recognize that proportional position control within the stagesdescribed herein can be achieved with the present invention by applyingPulse Width Modulated current to the coil of the solenoid 172. Theamount of proportional control thus achieved is limited by the springrates of each of the stage bias springs. The greater the spring rate,the greater the proportional control within the operating position orstage.

While the above-described embodiments of the sequential steppeddirectional control valve 110 have been found and are believed to beuseful and preferable, particularly in certain applications involving,for example, engine braking, skilled practitioners will recognize thatother combinations of elements, dimensions, or materials can beutilized, and other equipment applications can be realized, withoutdeparting from the invention claimed herein. Moreover, although certainembodiments of the invention have been described by way of example, itwill be understood by skilled practitioners that modifications may bemade to the described embodiments without departing from the scope ofthe invention, which is defined by the claims.

Having thus described exemplary embodiments of the invention, that whichis desired to be secured by Letters Patent is claimed below.

1. A sequential stepped directional control valve having at least threeoperating positions, in which one of the operating positions is aneutral position, comprising: (A) a hollow cage having an axialdirection and a radial direction, the cage being pierced by a cagepressure port, by a cage first stage port, by a cage second stage port,and by a cage tank port; (B) a spool adapted to fit snugly within thecage in which location the spool is moveable relative to the cage in adirection substantially parallel to the axial direction of the cage; (C)a single solenoid coil substantially annularly surrounding a single polepiece adapted to produce a magnetic force in response to an electricalcurrent applied to the solenoid coil; (D) a magnetically responsiveplunger attached to the spool, the plunger being magnetically responsiveto the magnetic force produced by the pole piece when electrical currentis applied to the solenoid coil; (E) one or more springs within thesequential stepped directional control valve directly or indirectlybiasing the spool; (F) wherein (1) when no electrical current is appliedto the solenoid coil, the springs within the sequential steppeddirectional control valve directly or indirectly bias the spool suchthat the spool is located in the neutral position; (2) when a firstpredetermined electrical current is applied to the solenoid coil, themagnetic force produced in the pole piece acts upon the plunger tocounteract the biasing caused by the springs directly or indirectly uponthe spool so as to cause the plunger to move to a position within thesequential stepped directional control valve such that the spoolattached to the plunger is located in a first stage operating positiondifferent from the neutral position; and (3) when a second predeterminedelectrical current is applied to the solenoid coil, the magnetic forceproduced in the pole piece acts upon the plunger to counteract thebiasing caused by the springs directly or indirectly upon the spool soas to cause the plunger to move to a position within the sequentialstepped directional control valve such that the spool attached to theplunger is located in a second stage operating position different fromthe neutral position and different from the first stage operatingposition; and (G) wherein (1) when the spool is in the neutral position,the spool acts to block hydraulic fluid under pressure from entering thesequential stepped directional control valve through the cage pressureport, the spool allows hydraulic fluid to flow through the cage firststage port into the sequential stepped directional control valve and thespool directs the hydraulic fluid entering the cage first stage portinto the sequential stepped directional control valve to flow out of thesequential stepped directional control valve through the cage tank port,and the spool allows hydraulic fluid to flow through the cage secondstage port into the sequential stepped directional control valve and thespool directs hydraulic fluid entering the second stage port into thesequential stepped directional control valve to flow out of thesequential stepped directional control valve through the cage tank port;(2) when the spool is in the first stage operating position, the spoolallows hydraulic fluid under pressure to flow through the cage pressureport into the sequential stepped directional control valve and the spooldirects the hydraulic fluid under pressure entering the cage pressureport to flow out of the sequential stepped directional control valvethrough the cage first stage port, the spool allows hydraulic fluid toflow through the cage second stage port into the sequential steppeddirectional control valve and the spool directs hydraulic fluid enteringthe second stage port into the sequential stepped directional controlvalve to flow out of the sequential stepped directional control valvethrough the cage tank port, and the spool blocks hydraulic fluid fromthe cage first stage port from flowing out of the sequential steppeddirectional control valve through the cage tank port; and (3) when thespool is in the second stage operating position, the spool allowshydraulic fluid under pressure to flow through the cage pressure portinto the sequential stepped directional control valve and the spooldirects the hydraulic fluid under pressure entering the cage pressureport to flow out of the sequential stepped directional control valvethrough the cage first stage port and through the cage second stageport, and the spool blocks hydraulic fluid from the cage first stageport and from the cage second stage port from flowing out of thesequential stepped directional control valve through the cage tank port.2. The sequential stepped directional control valve of claim 1 whereinthe spool moves from the first stage operating position to the secondstage operating position without ever being in the neutral position. 3.The sequential stepped directional control valve of claim 1 wherein thespool moves from the first stage operating position to the second stageoperating position without interrupting the flow of hydraulic fluidunder pressure from flowing through the cage pressure port into thesequential stepped directional control valve and without interruptingthe flow of hydraulic fluid under pressure out of the sequential steppeddirectional control valve through the cage first stage port.
 4. Thesequential stepped directional control valve of claim 2 wherein the cageis pierced by a cage pressure port in substantially the radialdirection, the cage is pierced by a cage first stage port insubstantially the radial direction, the cage is pierced by a cage secondstage port in substantially the radial direction, and the cage ispierced by a cage tank port in substantially the axial direction.
 5. Thesequential stepped directional control valve of claim 3 wherein the cageis pierced by a cage pressure port in substantially the radialdirection, the cage is pierced by a cage first stage port insubstantially the radial direction, the cage is pierced by a cage secondstage port in substantially the radial direction, and the cage ispierced by a cage tank port in substantially the axial direction.
 6. Thesequential stepped directional control valve of claim 4 wherein the polepiece is a pull pole piece, and the magnetic force produced in responseto electrical current being applied to the solenoid coil acts to pullthe plunger and the spool attached to the plunger toward the pull polepiece.
 7. The sequential stepped directional control valve of claim 5wherein the pole piece is a pull pole piece, and the magnetic forceproduced in response to electrical current being applied to the solenoidcoil acts to pull the plunger and the spool attached to the plungertoward the pull pole piece.
 8. The sequential stepped directionalcontrol valve of claim 6 wherein (A) the second predetermined electricalcurrent is greater than the first predetermined electrical current; (B)the plunger is closer to the pull pole piece when the spool attached tothe plunger is in the second operating position than when the spoolattached to the plunger is in the first operating position; and (C) theplunger is closer to the pull pole piece when the spool attached to theplunger is in the first operating position than when the spool attachedto the plunger is in the neutral position.
 9. The sequential steppeddirectional control valve of claim 7 wherein (A) the secondpredetermined electrical current is greater than the first predeterminedelectrical current; (B) the plunger is closer to the pull pole piecewhen the spool attached to the plunger is in the second operatingposition than when the spool attached to the plunger is in the firstoperating position; and (C) the plunger is closer to the pull pole piecewhen the spool attached to the plunger is in the first operatingposition than when the spool attached to the plunger is in the neutralposition.
 10. A sequential stepped directional control valve having atleast three operating positions, in which one of the operating positionsis a neutral position, comprising: (A) a hollow cage having an axialdirection and a radial direction substantially transverse to the axialdirection of the cage, the cage having an outer cage wall, the cagehaving in inner cage wall defining a hollow cage central passage, thecage being pierced by a cage pressure port connecting the outer cagewall and the cage central passage, by a cage first stage port connectingthe outer cage wall and the cage central passage, by a cage second stageport connecting the outer cage wall and the cage central passage, and bya cage tank port connecting the outer cage wall and the cage centralpassage; (B) a spool adapted to fit snugly within the cage in whichlocation the spool is moveable relative to the cage in a directionsubstantially parallel to the axial direction of the cage, the spoolhaving an axial direction substantially parallel to the axial directionof the cage and a radial direction substantially parallel to the radialdirection of the cage, the spool having a outer spool wall, the spoolfurther having a hollow central spool passage extending in a directionsubstantially parallel with the axial direction of the cage, the spoolbeing pierced by a spool first stage port connecting the outer spoolwall and the central spool passage, the spool being pierced by a spoolsecond stage port connecting the outer spool wall and the central spoolpassage, and the spool being pierced by a spool tank port connecting theouter spool wall and the central spool passage; (C) the spool having (1)a first spool cavity comprising a first radially inward indentation ofthe outer spool wall wherein, when the spool is within the cage, thefirst radially inward indentation of the outer spool wall and the innercage wall define a cavity between the outer spool wall and the innercage wall, and (2) a second spool cavity comprising a second radiallyinward indentation of the outer spool wall wherein, when the spool iswithin the cage, the second radially inward indentation of the outerspool wall and the inner cage wall define a cavity between the outerspool wall and the inner cage wall; (D) a single solenoid coilsubstantially annularly surrounding a single pole piece adapted toproduce a magnetic force in response to an electrical current applied tothe solenoid coil; (E) a magnetically responsive plunger attached to thespool, the plunger being magnetically responsive to the magnetic forceproduced by the pole piece when electrical current is applied to thesolenoid coil; (F) one or more springs within the sequential steppeddirectional control valve directly or indirectly biasing the spool inthe axial direction of the spool; (G) wherein (1) when no electricalcurrent is applied to the solenoid coil, the springs within thesequential stepped directional control valve directly or indirectly biasthe spool such that the spool is located in the neutral position; (2)when a first predetermined electrical current is applied to the solenoidcoil, the magnetic force produced in the pole piece acts upon theplunger to partially counteract the biasing caused by the springsdirectly or indirectly upon the spool so as to cause the plunger to moveto a position within the sequential stepped directional control valvesuch that the spool attached to the plunger is located in a first stageoperating position different from the neutral position; and (3) when asecond predetermined electrical current is applied to the solenoid coil,the magnetic force produced in the pole piece acts upon the plunger topartially or fully counteract the biasing caused by the springs directlyor indirectly upon the spool so as to cause the plunger to move to aposition within the sequential stepped directional control valve suchthat the spool attached to the plunger is located in a second stageoperating position different from the neutral position and differentfrom the first stage operating position; and (H) wherein (1) when thespool is in the neutral position (a) the spool blocks hydraulic fluidunder pressure from entering the sequential stepped directional controlvalve through the cage pressure port, (b) the spool first stage port isaligned with the cage first stage port and the spool tank port isaligned the with cage tank port, so that hydraulic fluid may flowthrough the cage first stage port and through the aligned spool firststage port into the central spool passage and from the central spoolpassage through the spool tank port and out of the sequential steppeddirectional control valve through the cage tank port aligned with thespool tank port, (c) the spool second stage port is aligned with thecage second stage port, so that hydraulic fluid may flow through thecage second stage port and through the aligned spool second stage portinto the central spool passage and from the central spool passagethrough the spool tank port and out of the sequential steppeddirectional control valve through the cage tank port aligned with thespool tank port; (2) when the spool is in the first stage operatingposition (a) the first spool cavity is simultaneously aligned with boththe cage pressure port and the cage first stage port, resulting inhydraulic fluid under pressure flowing into the sequential steppeddirectional control valve through the cage pressure port into the firstspool cavity, and hydraulic fluid under pressure in the first spoolcavity flowing out of the sequential stepped directional control valvethrough the cage first stage port with which the first spool cavity isaligned, (b) the spool first stage port is blocked by the cage innerwall, (c) the spool second stage port is aligned with the cage secondstage port and the spool tank port is aligned the with cage tank port,so that hydraulic fluid may flow through the cage second stage port andthrough the aligned spool second stage port into the central spoolpassage and from the central spool passage through the spool tank portand out of the sequential stepped directional control valve through thecage tank port aligned with the spool tank port; and (3) when the spoolis in the second stage operating position (a) the first spool cavity issimultaneously aligned with both the cage pressure port and the cagefirst stage port, resulting in hydraulic fluid under pressure flowinginto the sequential stepped directional control valve through the cagepressure port into the first spool cavity, and hydraulic fluid underpressure in the first spool cavity flowing out of the sequential steppeddirectional control valve through the cage first stage port with whichthe first spool cavity is aligned, (b) the second spool cavity issimultaneously aligned with both the cage first stage port and the cagesecond stage port, resulting in hydraulic fluid under pressure in thecage first stage port flowing into the second spool cavity, andhydraulic fluid under pressure in the second spool cavity flowing out ofthe sequential stepped directional control valve through the cage secondstage port with which the second spool cavity is aligned, and (c) thespool first stage port and the spool second stage port are blocked bythe cage inner wall.
 11. The sequential stepped directional controlvalve of claim 10 wherein the spool moves from the first stage operatingposition to the second stage operating position without ever being inthe neutral position.
 12. The sequential stepped directional controlvalve of claim 10 wherein the spool moves from the first stage operatingposition to the second stage operating position without interrupting theflow of hydraulic fluid under pressure from flowing through the cagepressure port into the sequential stepped directional control valve andwithout interrupting the flow of hydraulic fluid under pressure out ofthe sequential stepped directional control valve through the cage firststage port.
 13. The sequential stepped directional control valve ofclaim 11 wherein the cage is pierced by a cage pressure port insubstantially the radial direction, the cage is pierced by a cage firststage port in substantially the radial direction, the cage is pierced bya cage second stage port in substantially the radial direction, and thecage is pierced by a cage tank port in substantially the axialdirection.
 14. The sequential stepped directional control valve of claim12 wherein the cage is pierced by a cage pressure port in substantiallythe radial direction, the cage is pierced by a cage first stage port insubstantially the radial direction, the cage is pierced by a cage secondstage port in substantially the radial direction, and the cage ispierced by a cage tank port in substantially the axial direction. 15.The sequential stepped directional control valve of claim 13 wherein (A)the pole piece is a pull pole piece, and the magnetic force produced inresponse to electrical current being applied to the solenoid coil actsto pull the plunger and the spool attached to the plunger toward thepull pole piece; and (B) the springs within the sequential steppeddirectional control valve directly or indirectly bias the spool in theaxial direction of the spool in a direction away from the pull polepiece.
 16. The sequential stepped directional control valve of claim 14wherein (A) the pole piece is a pull pole piece, and the magnetic forceproduced in response to electrical current being applied to the solenoidcoil acts to pull the plunger and the spool attached to the plungertoward the pull pole piece; and (B) the springs within the sequentialstepped directional control valve directly or indirectly bias the spoolin the axial direction of the spool in a direction away from the pullpole piece.
 17. The sequential stepped directional control valve ofclaim 15 wherein (A) the second predetermined electrical current isgreater than the first predetermined electrical current; (B) the plungeris pulled closer to the pull pole piece when the second predeterminedcurrent is applied to the solenoid coil resulting in the spool attachedto the plunger being in the second operating position than when thefirst predetermined electrical current is applied to the solenoid coilresulting in the spool attached to the plunger being in the firstoperating position; and (C) the plunger is pulled closer to the pullpole piece when the first predetermined is applied to the solenoid coilresulting in the spool attached to the plunger being in the firstoperating position than when the spool attached to the plunger is in theneutral position.
 18. The sequential stepped directional control valveof claim 16 wherein (A) the second predetermined electrical current isgreater than the first predetermined electrical current; (B) the plungeris pulled closer to the pull pole piece when the second predeterminedcurrent is applied to the solenoid coil resulting in the spool attachedto the plunger being in the second operating position than when thefirst predetermined electrical current is applied to the solenoid coilresulting in the spool attached to the plunger being in the firstoperating position; and (C) the plunger is pulled closer to the pullpole piece when the first predetermined is applied to the solenoid coilresulting in the spool attached to the plunger being in the firstoperating position than when the spool attached to the plunger is in theneutral position.